1. Field of the Invention
The present invention is directed to a system and method for monitoring pre-ionization in a gas laser prior to firing the laser.
2. Prior Art
Slab type gas lasers, such as carbon dioxide (CO2) lasers, are widely used for industrial purposes and have developed into lightweight and economical devices for performing various industrial cutting, engraving and similar operations. Metal plates or slabs are utilized as electrodes having a large contact area and forming a chamber between the plates. Typically, cooling is provided within the slabs, such as a water cooling system, to prevent overheating of the laser.
In order to maintain conditions in which gas lasers are ready to fire a laser beam, the plasma in the firing chamber is pre-ionized, also known as “tickling” the laser. In order to ready the plasma to maintain the laser in a condition in which it is ready to fire quickly, short radio frequency pulses are sent to the laser tube. In this condition, the laser tube is ionized, but not ionized to such an extent that the laser actually emits a laser beam. Pre-ionizing the laser tube also creates a better Radio Frequency (RF) load matching between a Radio Frequency Power Amplifier (RFPA) and the laser allowing for improved rise time and fall time of the emitted laser beam.
It has been found that maintaining the proper pre-ionized state presents several challenges. If the pulses are not sufficiently wide, there will be delay in reaching an ionized state for firing the laser. Moreover, if there is a load mismatch between the laser tube and the RFPA, damage can be done to the RFPA from excess voltage feedback. Furthermore, if the pulses are too long, the laser may fire prematurely. Therefore, maintaining the proper pulsing prevents load mismatching and inaccurate laser firing.
In order to overcome these problems related to maintaining a pre-ionized state ready for firing of the laser, systems have been developed in an attempt to monitor the condition of the laser tube. In one system, the reflected RFPA power is monitored to determine if the laser is sufficiently pre-ionized. Although such a system may prevent some of the problems related to damage and inaccurate firing, the system has several drawbacks. The reflected RF power is an indirect measurement and also may not be sufficiently accurate to maintain the proper pre-ionized condition. Moreover, such a system is expensive and complicated and adds unwanted cost to the laser.
It can be seen that a new and improved system for maintaining the plasma in a laser at a pre-ionized condition is required. Such a system should be accurate and provide for maintaining a proper state of readiness to fire the laser tube. Moreover, such a system should provide for accurate, inexpensive and direct measurement of the conditions of the laser tube and controls to maintain the proper RFPA power and pulse width directed at the laser chamber. The present invention addresses these as well as other problems related to maintaining and monitoring pre-ionized conditions for gas slab lasers.